In communications technology, particularly in mobile radio technology, a circuit arrangement frequently has two circuit blocks which operate at two different supply voltages. Such a circuit arrangement is a “mixed signal” circuit, for example, which has at least one analogue circuit block and at least one digital circuit block. These circuit blocks are often integrated in a single semiconductor component. The mixed-signal circuit usually requires two supply voltages at different levels, because the analogue circuit block normally operates at a higher supply voltage in comparison with the digital circuit block.
When the supply voltage for a digital circuit block is turned on, an output signal from a logic gate which the circuit block contains, for example a multivibrator or flip-flop, takes a random logic value. The output signal is initially therefore randomly set at the logic value “1”, which usually corresponds to a high voltage level, or at the logic value “0”, which usually corresponds to a low voltage level. As a result, it is necessary to put the logic gate in the circuit block into a desired or stipulated initial state using a reset signal. The predetermined initial state ensures reliable operation for the digital circuit block.
The reset to an initial state is also called “power-on reset”. Normally, the reset signal is triggered when, during a turn-on operation, the voltage level of a rising supply voltage has reached or exceeded a particular threshold value. Such a power-on reset is likewise necessary when the supply voltage has dropped below the threshold value for a certain period. In this case, reliable operation of the circuit block is no longer ensured.
The fact that the circuit blocks operate at different supply voltages and that the supply voltages can be turned on and off independently of one another means that reliable operation of the circuit arrangement is not always ensured. This is the case particularly when a first circuit block actuates operation of a second circuit block. In a mixed-signal circuit, the analogue circuit block is started up, i.e. activated, and turned off, or deactivated, by the digital circuit block.
There is a possible operating state for the circuit arrangement in which the second circuit block is already supplied with a supply voltage which is sufficient for operation, whereas the operation of the first circuit block is not reliably ensured because the voltage level of the corresponding supply voltage is still too low. On the other hand, the situation may arise in which the first circuit block is already operating reliably, whereas an excessively low voltage level of the supply voltage is being applied to the second circuit block. This means that it is necessary when starting up the entire circuit arrangement to wait until the various supply voltages for the two circuit blocks have reached a respective sufficiently high voltage level.
To ensure reliable operation of the circuit arrangement, a reset signal for the first circuit block therefore cannot appear until after a certain waiting period. However, determining the waiting period requires a well-defined order for turning on the supply voltage as well as a well-known time profile for the voltage levels when turning on the supply voltage. This cannot always be ensured.